Method for recycling foamed polystyrol resin

ABSTRACT

Provided are a method for recycling expanded polystyrene, comprising steps of: reducing a volume of the expanded polystyrene  110 ; dissolving the volume-reduced polystyrene in a solvent  130 ; and extruding the dissolved expanded polystyrene  160 ; and a separation and recovery apparatus of a polystyrene solution used in the extruding step. The present invention makes it possible to recycle expanded polystyrene while minimizing the problems of the conventional methods such as difficulty in the removal of foreign matters and the reduction of the molecular weight caused by heating in the extrusion step in the heat melting method, and discharge of the vaporized solvent out of the system together with a gas emitted during the volume-reduction step and the loss of the solvent thereby in the dissolution method.

TECHNICAL FIELD

The present invention relates to a recycling method of expandedpolystyrene. More specifically, the invention relates to a method ofrecycling the high-quality resin for expanded polystyrene from wastematerials of molded or formed products of expanded polystyrene. Thepresent invention also pertains to a separation and recovery apparatusof a polystyrene solution for separating and recovering a solvent, froma polystyrene solution in which expanded polystyrene has been dissolvedin the solvent.

RELATED ART

Expanded polystyrene is used in large quantities as various packagingmaterials of household electric appliances, and also as buildingmaterials. Recycle of expanded polystyrene chips or used molded orformed products of expanded polystyrene is becoming a big problem inview of environmental conservation and economical efficiency.

Upon recycling use of expanded polystyrene, there are usually twoproblems. One is that expanded polystyrene is bulky and transportationof it as is requires a high cost. Volume reduction of expandedpolystyrene prior to transportation is desired. The other one is thatquality deterioration occurs owing to dusts or foreign matters mixedduring the regeneration procedure of expanded polystyrene and reductionof molecular weight caused by heating mainly during the treatment.

For the regeneration of expanded polystyrene, two methods, whenclassified roughly, has been proposed: heat melting method anddissolution method by solvent. These methods however are notsatisfactory for overcoming the above-described problems.

In the heat melting method, expanded polystyrene waste which has beenbulky owing to expansion is melted by heating to reduce its volume intoa resin mass (ingot) having a higher density, crushing thevolume-reduced resin into pieces of a proper size, throwing them in anextruder, and extruding them into pellets while heating. At the volumereduction by heat melting, the apparent density of the resin becomesabout 1.0 g/cm³. As a variation of the method, operations from volumereduction by heat melting to pelletization are carried out in one stepin an extruder after crushing the expanded polystyrene waste ifnecessary.

In the heat melting method, foreign matters may be removed using ascreen in the final step of extrusion into pellets, but the filtrationof the semi-solid resin does not completely remove therefrom the foreignmatters. It is therefore necessary to remove the foreign matters fromexpanded polystyrene before heat melting. Since foreign matters mixed inthe expanded polystyrene waste must be removed manually, it is difficultto remove foreign matters in the heat melting method.

Moreover, in the heat melting method, reduction of the molecular weightof the resin tends to occur due to heating. It leads to a qualitydeterioration of the molded or formed products of recycled expandedpolystyrene such as reduction of tensile strength or impact resistance.

On the other hand, in the dissolution method by solvent, expandedpolystyrene is volume-reduced by dissolving it in an organic solvent,pouring the resulting solution into an extruder, mixing the solutionunder heat to melt the resin while evaporating and removing the solvent,and preparing the extruded product in the form of pellets. As thesolvent, chlorinated hydrocarbon, limonene, THF and the like areproposed. Examples of the dissolution method by solvent include those asdisclosed in Japanese Patent Provisional Publication No. 10-330530 andJapanese Patent Provisional Publication No. 2000-334738.

In the dissolution method by solvent, it is possible to filter the resinin the dissolved liquid form, so the method is suited for the removal offoreign matters. In addition, deterioration in quality of the recycledproducts owing to reduction of the molecular weight occurs less, becauseheating is conducted at not so high temperature. It, however, requires astep of efficiently removing the solvent from a solution containing theresin dissolved therein. In addition, since the method uses a solvent,an extra facility or person handling the solvent is required throughoutthe steps for recycling of expanded polystyrene. In particular, whenextrusion is conducted at another plant after volume reduction using anorganic solvent, transportation of the solvent is inevitable; this leadsto a rise in the transportation cost.

Expanded polystyrene contains therein a large amount of a gas asbubbles. The gas is emitted upon dissolution. Together with the emittedgas, a vaporized solvent also flows out of the apparatus. When a solventis used in the volume reducing step of expanded polystyrene, the solventis released from the apparatus together with a large amount of the gas.When the solvent has a low-boiling-point, loss of the solvent cannot beneglected. Also from the viewpoint of environmental conservation,release of such a vaporized solvent out of the apparatus is notpreferred and must be prevented. A solvent recovery apparatus thereforebecomes necessary in order to prevent the loss of the solvent.Dissolution of expanded polystyrene on a too small scale is thereforenot economical from the viewpoints of facilities and management andcannot be realized.

Dissolution of expanded polystyrene at a place where the waste occurs isproposed because it is bulky and has low transport efficiency (JapanesePatent Provisional Publication No. 11-181144 and Japanese PatentProvisional Publication No. 2000-154275). The method is howeveraccompanied with the problems that the small recycling scale is notpractical from the viewpoints of facilities and management andsimultaneous transportation of the solvent raises the transportationcost.

Regardless of the specific recycling method, there is generally a demandfor the reduction in the volume of expanded polystyrene fortransportation or storage of it. Long distance transportation ofexpanded polystyrene used as packaging materials or expandedpolyethylene waste such as chips remaining after production of itsmolded or formed products inevitably heightens the recycling cost. Anobject of the present invention is to overcome the problems which theprior art has.

An apparatus for separating and recovering polystyrene and a solventfrom a polystyrene solution used in the dissolution method by solvent isdisclosed in Japanese Patent Provisional Publication No. 2000-7822. Asillustrated in the front view of FIG. 9, the apparatus comprises anextrusion portion having a main shaft screw 42 built in a barrelequipped with a heater; a driving device 43 for rotating the main shaftscrew 42 which device is attached at the base end of the main shaftscrew 42 to adjust the shape or pitch of the main shaft screw 42 and byturning the screw, to move the resin solution from the base end to thetip while pressurizing, heating, melting and kneading the resin solutionand vaporizing the solvent; a resin solution inlet 44 disposed in thevicinity of the base end of the main shaft screw 42; and an extrusionportion 4 attached to the tip for extruding the molten resin. Theextrusion portion is equipped with a vaporized solvent recovery portion46 for collecting, degassing and recovering the solvent separated byvaporization; and a liquefied solvent recovery portion 47 disposeddownstream of the vaporized solvent recovery portion 46 for cooling andliquefying the vaporized solvent by a heat exchanger.

The above-described separation and recovery apparatus of polystyrene andsolvent from the polystyrene solution as disclosed in Japanese PatentProvisional Publication No. 2000-7822 however involves the followingproblems.

(1) The polystyrene solution advances in a barrel (cylinder) while beingkneaded by a screw. The polystyrene solution falling in the grooves ofthe screw however must inevitably advance along the screw shaft and isnever stirred. Accordingly, the heat from a heater attached to the wallsurface of the barrel is not effectively transferred to the polystyrenesolution, which disturbs the promotion of the vaporization of thesolvent.

(2) The space between the inner wall of the cylinder and the screwbecomes smaller as the solution approaches the tip portion. The gasgenerated by heating of the solvent must inevitably advance toward theoutlet while drawing a spiral trace along the screw shaft so that thegas cannot be eliminated easily and is discharged from the outlet of thecylinder without sufficiently accomplishing the separation between thepolystyrene and the solvent. The object of the present invention is toovercome the above-described problems of the conventional art. Anotherobject of the present invention is to provide a separation and recoveryapparatus of a polystyrene solution facilitating the promotion of thevaporization of a solvent and permitting removal of the vaporized gasefficiently.

SUMMARY OF THE INVENTION

The present invention provides a method for recycling expandedpolystyrene comprising steps of: reducing a volume of the expandedpolystyrene; dissolving the volume-reduced expanded polystyrene in asolvent; extruding the dissolved expanded polystyrene; and producingrecycled expanded polystyrene from the extruded product. According tothe present invention, in the recycling method of expanded polystyrene,the above-described reducing step preferably comprises volume-reductionby mechanical compression and/or volume-reduction by partial melting ata temperature not greater than 200° C. The solvent used in thedissolving step is preferably a solvent having a boiling point notgreater than 150° C., with methylene chloride being especially suited.It is more preferred that the solvent comprises unsaturated hydrocarbonhaving 5 to 7 carbon atoms and/or epoxide.

Moreover, according to the present invention, the method furthercomprises a transportation step for transporting the volume-reducedexpanded polystyrene after the reducing step to carry out the followingsteps at another place. It is also possible that the method furthercomprises a second transportation step for transporting the extrudedproduct after the extruding step to produce recycled expandedpolystyrene at another place.

In the method of the present invention, the step before extrusion inrecycling of the expanded polystyrene is classified into two steps, thatis, a step of reducing a volume and a step of dissolving by solvent, inorder to overcome the defects of the heat melting method and dissolutionmethod by solvent. Described specifically, reduction of the molecularweight of the resin occurs in the conventional heat melting methodbecause the resin is susceptible to be heated locally in the extrusionstep and is oxidized by the air involved in the resin. It has been foundthat to overcome the above-described problems, the dissolution method bysolvent is excellent in the quality of the product available therebyfrom the synthetic viewpoint, because it permits smooth heat transferand does not involve the air in the resin, and it permits removal offoreign matters by filtration.

The dissolution method by solvent is accompanied with such drawbacksthat specialized technology or knowledge is necessary for handling ormanagement of solvents, the solvent is lost together with an expandingagent emitted upon the dissolving step, and an extra facility for thesolvent is necessary. Operation on a small scale is not economical and acertain scale is necessary for carrying out the dissolving step. Volumereduction is inevitable for suppressing the transportation cost of theexpanded polystyrene. In the conventional heat melting method,deterioration in the quality of the regenerated product occursinevitably. When the solvent is transported to a site where the wastematerial occurs and when volume reduction is conducted there, areduction in the transportation cost of the solvent cannot be attained.In addition, there is a problem in handling or management of the solventduring the transportation.

With the foregoing in view, it has been found that the method of thepresent invention which does not reduce the molecular weight of theresin and can be performed in a solventless manner is most suited forthe volume reduction of the expanded polystyrene and the methodcomprising, after the volume reduction, dissolving the resin in asolvent and treating the solution is an efficient recycling method ofexpanded polystyrene waste.

The first separation and recovery apparatus of a polystyrene solutionaccording to the present invention is an apparatus wherein thepolystyrene solution obtained by dissolving expanded polystyrene in asolvent is made to advance in a cylinder while being heated, so that thesolvent is vaporized for recovery and the polystyrene is separatedtherefrom, comprising: a mixing shaft disposed in the cylinder,comprising mixing impellers being adjacent each other in a shaftdirection and being shifted in a circumferential direction.

The second separation and recovery apparatus of a polystyrene solutionaccording to the present invention is an apparatus wherein thepolystyrene solution obtained by dissolving expanded polystyrene in asolvent is made to advance in a cylinder while being heated, so that thesolvent is vaporized for recovery and the polystyrene is separatedtherefrom, comprising: a cylinder comprising a first-stage cylindermember and a second-stage cylinder member; and an mixing shaft disposedin the first-stage cylinder member, comprising mixing impellers beingadjacent each other in a shaft direction and being shifted in acircumferential direction, wherein the second-stage cylinder membercomprising a screw shaft is disposed at right angles with thefirst-stage cylinder member.

The third separation and recovery apparatus of a polystyrene solutionaccording to the present invention is an apparatus wherein thepolystyrene solution obtained by dissolving expanded polystyrene in asolvent is made to advance in a cylinder while being heated, so that thesolvent is vaporized for recovery and the polystyrene is separatedtherefrom, comprising: a cylinder comprising a first-stage cylindermember, a second-stage cylinder member, and a third-stage cylindermember; an mixing shaft disposed in the first-stage cylinder member,comprising mixing impellers being adjacent each other in a shaftdirection and being shifted in a circumferential direction, wherein thesecond-stage cylinder member comprising a screw shaft is disposed atright angles with the first-stage cylinder member, and the third-stagecylinder member comprising a screw shaft is disposed at right angleswith the second-stage cylinder member.

In the first separation and recovery apparatus of a polystyrene solutionaccording to the present invention, an mixing shaft comprising mixingimpellers being adjacent each other in the shaft direction and beingshifted in a circumferential direction is disposed in the cylinder. Inthe apparatus, therefore, the polystyrene solution can be mixedefficiently and heat conduction from a heaters is promoted, making itpossible to accelerate the vaporization of the solvent from thepolystyrene solution.

The gas obtained by the vaporization of the solvent can be transferredalmost directly along the shaft direction of the cylinder so thatpolystyrene has been separated from the solvent sufficiently when thegas flows out of the outlet of the cylinder.

In the second separation and recovery apparatus of a polystyrenesolution according to the present invention, the second-stage cylindermember equipped with a screw shaft is disposed at right angles with thefirst-stage cylinder member. This structure makes it possible to cut andextrude the molten polystyrene, which has been fed from the first stagecylinder member, by the screw of the second-stage cylinder member,thereby promoting the vaporization of the remaining solvent.

In the third separation and recovery apparatus of a polystyrene solutionaccording to the present invention, the third-stage cylinder memberequipped with a screw shaft is disposed at right angles with thesecond-stage cylinder member. This structure makes it possible to cutand extrude the molten polystyrene, which has been fed from the secondstage cylinder member, by the screw of the third-stage cylinder member,thereby promoting the vaporization of the remaining solvent further.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the method for recycling expandedpolystyrene according to the present invention;

FIG. 2 is a longitudinal cross-sectional view illustrating a separationand recovery apparatus of a polystyrene solution according to a firstembodiment of the present invention;

FIG. 3 is a longitudinal cross-sectional view of a separation andrecovery apparatus of a polystyrene solution according to a secondembodiment of the present invention;

FIG. 4 is a plan cross-sectional view of a separation and recoveryapparatus of a polystyrene solution according to the second embodimentof the present invention;

FIG. 5 is a view taken along a line of A-A of FIG. 4;

FIG. 6 is a view illustrating the molten polystyrene whittled away by aspiral mixing impeller continuously;

FIG. 7 is a partial plan cross-sectional view illustrating thesecond-stage and third-stage cylinder member of a separation andrecovery apparatus of a polystyrene solution according to a thirdembodiment of the present invention;

FIG. 8 is a view taken along a line B-B of FIG. 7; and

FIG. 9 is a front view illustrating the conventional polystyreneseparation and recovery apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of the present invention will hereinafter be described.

The volume reducing step of the invention comprises volume reduction bymechanical compression and/or volume reduction by partially melting at atemperature not greater than 200° C.

The term “volume reduction by mechanical compression” as used hereinmeans crushing expanded polystyrene waste optionally to a sizepermitting introduction of it in the subsequent step, followed bycompression by a pressing machine or an extruder to partially melt thecrushed polystyrene. The term “volume reduction by partially melting ata temperature not greater than 200° C.”, means partially melting theexpanded polystyrene while applying external heat, at a temperature notgreater than 200° C. during the above-described volume reduction bymechanical compression. By the volume reduction, the apparent density ofthe expanded polystyrene can be reduced from 0.02 g/cm³, which is atypical density of expanded polystyrene, to about 0.1 to 0.5 g/cm³,preferably about 0.2 to 0.4 g/cm³.

The volume reduction by mechanical compression according to the presentinvention may partially melt the expanded polystyrene by internal heatsuch as frictional heat generated upon crushing or compression. However,an increase of the temperature is small in this case compared with theheat melting method and reduction of the molecular weight can beminimized.

Melting of expanded polystyrene usually leads to a rise in its apparentdensity and advance of volume reduction. However, melting at a too hightemperature may cause deterioration of the polystyrene owing toexcessive reduction of the molecular weight. The volume reduction bymechanical compression utilizing mainly the internal heat such asfrictional heat as in the present invention is advantageous in view ofthe quality of recycled polystyrene.

When the reduction of the molecular weight is allowed to some extent,the step of volume reducing may be carried out by compressing thepolystyrene while partially melting by external heating. In this case,however, melting is preferably performed at a temperature not greaterthan 200° C., more preferably not greater than 180° C. in order toprevent drastic reduction of the molecular weight. External heating maysometimes cause unequal distribution of heat or make partially hotportions. In the present invention, the temperature is based on that ofa portion having the highest temperature.

The step of volume reducing can yield a compressed resin ingot. In thestep, no solvent is used so that handling of the apparatus and the stepitself are relatively easy. In the place where expanded polystyrenewaste, which will be a raw material, occurs, volume reducing step can beeasily carried out even on a small scale.

The expanded polystyrene having a reduced volume is then dissolved in asolvent. The dissolving step can be carried out in a plant where thevolume reduction is conducted. However the step can be carried out moreefficiently by transporting the expanded polystyrene to another plant,because the expanded polystyrene has increased transport efficiency bythe volume reduction. And then an adequate amount of the expandedpolystyrene is collected, they can be dissolved en masse. Asolvent-using process requiring special management and equipment forhandling the solvent can be carried out efficiently at less frequency infewer places.

In the dissolving step, since the expanded polystyrene has already areduced volume, the amount of the solvent released from the apparatusupon dissolution can be decreased to the 1/10 to 1/50 of a solvent usedin the recycle of expanded polystyrene not subjected to volumereduction. This makes it possible to reduce the load on a recoveryapparatus of the solvent from the released gas, or reduce the loss ofthe solvent upon the discharge.

In the dissolving step of the present invention, the volume-reducedexpanded polystyrene can be dissolved in a solvent by pouring it in thesolvent and mixing the mixture. The volume-reduced expanded polystyrenecan also be dissolved in the solvent by using a kneader or by puttingthe expanded polystyrene crushed into a proper size and the solvent inan extruder. By the dissolving step, foreign matters contained in theresin can be filtered off, whereby the foreign matters can be removedeasily compared with the heat melting method.

In the dissolving step, any solvent can be used as far as it candissolve therein the expanded polystyrene. Examples of the solventinclude chlorine-based solvents such as methylene chloride andtrichloroethylene, limonene, and THF. Solvents having a relatively lowboiling point, especially solvents having a boiling point not greaterthan 150° C. are preferred in view of the subsequent solvent evaporationstep which can be performed at a low temperature. Methylene chloridehaving a boiling point of about 40° C. is most suited because it is alsoexcellent in dissolving power.

When methylene chloride is used as the solvent, addition of anunsaturated hydrocarbon having 5 to 7 carbon atoms and/or epoxide mayprevent deterioration of the solvent even if evaporation andcondensation of the solvent are conducted in repetition. In particular,the solvent tends to undergo thermal decomposition, since the extrusionstep which will be described later is conducted under relatively hightemperature.

Epoxide having a boiling point of from 30 to 90° C. is preferably used.Specific examples include propylene oxide, butylene oxide andtetrahydrofuran. These epoxides may be added either singly or incombination with two or more of them. Its amount is from 0.1 to 1.0weight %, preferably from 0.2 to 0.5 weight %, based on the totalsolvent weight including methylene chloride and additives such asepoxide. When the amount of the epoxide is too small, it may noteffectively maintain the stability of methylene chloride. Too largeamounts of the epoxide may be economically disadvantageous.

Examples of the unsaturated hydrocarbon having 5 to 7 carbon atomsinclude 2-pentene, 2-methyl-2-butene, 2-hexene and 2-heptene. Theunsaturated hydrocarbons having 5 to 7 carbon atoms can be added eithersingly or in combination with two or more of them. The amount may befrom 10 to 200 ppm, preferably from 50 to 100 ppm, based on the totalsolvent weight including methylene chloride and additives such asunsaturated hydrocarbons. When the amount of the unsaturated hydrocarbonhaving 5 to 7 carbon atoms is less than 10 ppm, it may not contribute tothe decomposition suppressive effect of methylene chloride. Amountsexceeding 200 ppm may be, on the other hand, economicallydisadvantageous, because improvement in the decomposition suppressiveeffect proportional to such an amount cannot be expected so much.

Removal of the solvent by evaporation and production of pellets may becarried out by the following steps of evaporating the solvent by mixingwhile feeding a heated extruder or the like with a solution of theexpanded polystyrene, and extruding the resin into pellets, beads orneedles. In order to remove the solvent by evaporation, heat may beapplied from the outside. For preventing the reduction of the molecularweight of the resin, it is preferred to adjust the temperature to 200°C. or less, especially 180° C. or less.

By extruding the expanded polystyrene dissolved in a solvent, reductionof the molecular weight of the resin can be suppressed and the strengthof the recycled product can be maintained at a high level compared withthat when the heat melting method is employed.

For the removal of the solvent by evaporation, a separation and recoveryapparatus of a polystyrene solution which will be described later can beused. Since the separation and recovery apparatus of a polystyrenesolution according to the present invention is, in particular, capableof promoting vaporization of the remaining solvent. The apparatus ispreferably employed for the removal of the solvent by evaporation. Inaddition, the resin can be extruded into strands by using the apparatus.

During the extruding step of the expanded polystyrene dissolved in asolvent, the solution of the expanded polystyrene is evaporated byheating. The solvent recovered by the cold condensation of the resultingvapor can be provided for re-use.

The resulting extruded product can be then optionally transferred toanother plant. The product is impregnated with an expanding agent toregenerate expanded polystyrene. Preferably at a plant near a consumingplace, expansion and molding of the expanded polystyrene which has beenrecycled and impregnated with an expanding agent can be carried out. Anexpanded polystyrene molded product can be obtained as a recycledproduct. The expanding agent and expansion and molding method may besimilar to those employed for polystyrene which is not a recycledproduct.

As illustrated in FIG. 1, the volume reducing step and the dissolvingstep can be carried out at different places according to one embodimentof the present invention. So, a station 110 for the volume reducing stepwhich requires no solvent and can be easily managed may be provided neara place 100 where expanded polystyrene waste occurs. The station 110 ofthe volume reducing step can also be set up at a site such as plant,mass retailer, wholesaler, distribution basic point and the like wherethe waste occurs 200. The station can be equipped with a crusher, anextruder, a compressor and the like necessary for the volume reductionof the expanded polystyrene waste. As a result, the transport distanceof the waste from the place 100 where the expanded polystyrene wasteoccurs to the station 110 of the volume reducing step is relativelyshort. The expanded polystyrene which has a reduced volume and thereforehas merits in transport and storage is gathered in a plant 250 where thedissolving step 130 is carried out. In the dissolving step which needsmanagement of a solvent, economy of scale can be gained easily bygathering the expanded polystyrene and treating them en masse. After thedissolving step, a liquid resin composition is produced. The compositioncan be provided with treatment 140 such as removal of foreign mattersthrough filtration easily. After the removal of the solvent byevaporation 150, the resin is extruded (160).

At the same plant 250 or different plant 300, the pellets obtained byextrusion can be impregnated with an expanding agent. By expanding andmolding of the resulting pellets or beads at a plant 350 near theconsuming region, a new molded product 170 can be obtained. It isneedless to say that steps from the dissolving step 130 to the expandingand molding 350 can be carried out in one place.

After the dissolving step, the polystyrene can be taken out from thepolystyrene solution containing pellets of the polystyrene dissolvedtherein. The solvent is then removed. The separation and recoveryapparatus of the polystyrene solution will next be described based onsome embodiments.

FIG. 2 is a longitudinal cross-sectional view illustrating theseparation and recovery apparatus of a polystyrene solution according tothe first embodiment of the present invention.

The separation and recovery apparatus 1 of a polystyrene solution iscomposed mainly of a cylinder member 2, an mixing shaft 4 having aplurality of mixing impellers 3, which shaft is inserted in the cylindermember 2, and a plurality of heaters 5 attached to the outer peripheralsurface of the cylinder member 2 along the lengthwise direction thereof,and a driving device of the mixing shaft 4.

The separation and recovery apparatus 1 of a polystyrene solution willnext be described more specifically. The mixing shaft 4 is rotatablysupported, at one end portion thereof, by a support 6 disposed outsideof the cylinder member 2 via a bearing 7. The other end portion is notsupported and is a free end 4 a. At the outside portion of the support 6of the mixing shaft 4, a sprocket 8 for rotating the mixing shaft 4 isattached. A driving chain 11 connects between the sprocket 8 and anothersprocket 10 attached to the rotating shaft of an electric motor 9disposed in parallel with the mixing shaft 4, and rotation of the mixingshaft 4 is driven by the electric motor 9.

At the end of the cylinder member 2 on the side of the support 6, agland packing 12 is attached so that the polystyrene solution to bepoured inside of the cylinder member 2 does not leak from the spacebetween the mixing shaft 4 and cylinder member 2. At a position near thegland packing 12 of the cylinder member 2, an inlet 13 for thepolystyrene solution is arranged. To the inlet 13, a pouring pipe 14 isconnected. By a monopump 15 connected to the pouring pipe 14, thepolystyrene solution (obtained by dissolving expanded polystyrene in asolvent such as methylene chloride) stored in the polystyrene solutiontank 16 is poured inside of the cylinder member 2.

Near the end portion opposite to the portion to which the gland packing12 has been attached of the cylinder member 2, a vent tube 17 forremoving the gas is connected to remove the gas generated by thevaporization of the solvent in the polystyrene solution.

At the end portion of the mixing shaft 4 of the cylinder member 2 on theside of the free end 4 a, a die 18 for extruding the molten polystyreneafter the vaporization of the solvent from the polystyrene solution isdisposed.

The polystyrene solution poured in the cylinder member 2 is transferredin the cylinder member 2 toward the side of the die 18 under apredetermined pressure, which can be controlled by the monopump 15,while being heated by a plurality of heaters 5 and being mixed with themixing impellers 3.

When the polystyrene solution is heated by the heaters 5, the solventbecomes a gas by vaporization. The gas is separated from the polystyrenesolution, cooled and then recovered. The polystyrene separated from thesolvent is melted by heating.

The plurality of mixing impellers 3 attached to the mixing shaft 4 maybe positioned so that any two mixing impellers 3 adjacent each other inthe shaft direction are shifted by the ¼ circumference in thecircumferential direction. The mixing impellers 3 may be disposed at aproper distance while being shifted in the circumferential direction ofthe mixing shaft 4 not only by the ¼ circumference, but also by the ⅙circumference, by the ⅛ circumference or the like.

The mixing impellers 3 are attached to the mixing shaft 4, with agradient toward the face running along the axis of the mixing shaft 4.Moreover, the mixing impellers 3 are preferably attached to wrap acertain length of the shaft in the shaft direction.

Since the mixing impellers 3 are attached to the mixing shaft asdescribed above, the polystyrene solution in the cylinder member 2advances in the cylinder member 2 without leaving an unstirred portion.Heat can be therefore efficiently conducted from the heaters 5 to thepolystyrene solution and vaporization of the solvent from thepolystyrene solution can be accelerated.

In a mixing structure in which mixing can be carried out by rotating themixing shaft equipped with the mixing impellers, an adequate distancebetween any two adjacent mixing impellers enables the gas to reach thegas outlet very easily compared with the conventional apparatus adoptinga screw shaft in which a vaporized gas passes through a spiral routealong the screw and reaches the gas outlet.

The gas generated by the vaporization of the solvent moves almostdirectly along the mixing shaft 4 toward the vent tube 17 for gasremoval, while flowing through the space between the polystyrenesolution, which space is formed by the rotation of the mixing impellers3. Vaporization of the solvent therefore occurs actively, making itpossible to minimize a ratio of the remaining solvent in the resin.

The molten polystyrene after the solvent is vaporized and separated fromthe polystyrene solution reaches the die 18. By the pressure applied tothe polystyrene solution, it is extruded outside the cylinder member 2,as strands, from a plurality of through-holes made in the die 18.

Another separation and recovery apparatus of a polystyrene solutionaccording to the second embodiment of the present invention willhereinafter be described based on FIG. 3.

FIG. 3 is a longitudinal cross-sectional view of the separation andrecovery apparatus of the polystyrene solution according to the secondembodiment of the present invention; FIG. 4 is a plan cross-sectionalview of the separation and recovery apparatus of the polystyrenesolution according to the second embodiment of the present invention;and FIG. 5 is a view taken along a line A-A of FIG. 4.

The separation and recovery apparatus of a polystyrene solution issimilar to the separation and recovery apparatus 1 of a polystyrenesolution as described in the first embodiment of the present inventionexcept that a second-stage separation and recovery structure of apolystyrene solution is installed thereto. The members corresponding tothe first-stage separation and recovery apparatus 1 of a polystyrenesolution will be identified by the same reference numerals as used inFIG. 2 and detailed descriptions will be omitted.

In the separation and recovery apparatus of a polystyrene solutionaccording to the second embodiment, the die 18 at the end portion on theexhaust side of the first-stage cylinder member 2 is replaced by acylinder member 22 of the second-stage separation and recovery structure21 of a polystyrene solution to connect them at right angles. The moltenpolystyrene discharged from the first-stage cylinder member 2 is made toenter into the cylinder member 22 from the upstream of the second-stagecylinder member 22 in the lengthwise direction thereof.

The second-stage separation and recovery structure 21 of a polystyrenesolution is composed mainly of the cylinder member 22, an mixing shaft(screw shaft) 24 being inserted in the cylinder member 22 and having aspiral mixing impeller 23, a plurality of heaters 25 attached to theouter peripheral surface of the cylinder member along the lengthwisedirection thereof, and a driving device of the mixing shaft 24.

The driving device of the mixing shaft 24 has the same structure as thatof the driving device of the mixing shaft 4 of the first-stageseparation and recovery apparatus 1 of a polystyrene solution so that itis not illustrated.

This separation and recovery apparatus of a polystyrene solution isdesigned to have a higher solvent separating capacity than that of theseparation and recovery apparatus of a polystyrene solution according tothe first embodiment. The mixing impeller 23 has a spiral form and asillustrated in FIG. 6, the molten polystyrene entering the second-stagecylinder member 22 from the first-stage cylinder member 2 is whittledaway by the spiral mixing impeller 23 continuously and a whittledportion 101 extends in strands form.

The second cylinder member is designed to have an inner diameter largeenough not to fill it with the molten polystyrene flowing from thefirst-stage cylinder member into the second-stage cylinder member. Thisenables the extension of the whittled portion 101 in the strands form,to increase the surface area of the molten polystyrene and to promotethe vaporization of the solvent.

The gas generated by the vaporization of the solvent is discharged byits own pressure through the vent tube 25 disposed on the upstream sideof the connection between the second-stage cylinder member 22 and thefirst-stage cylinder member 2.

The molten polystyrene after vaporization and separation of the solventtherefrom advances in the second-stage cylinder member 22 and extrudedoutside, as strands form, from a plurality of through-holes of the die27 disposed at the tip of the cylinder 22.

A separation and recovery apparatus of a polystyrene solution accordingto the third embodiment of the present invention will hereinafter bedescribed based on FIG. 7.

FIG. 7 is a plan cross-sectional view illustrating the second-stage andthird-stage of the separation and recovery apparatus of a polystyrenesolution according to the third embodiment of the present invention, andFIG. 8 is a view taken along a line B-B of FIG. 7.

The separation and recovery apparatus of a polystyrene solution has theseparation and recovery structure 21 of a polystyrene solution asdescribed in the second embodiment of the present invention and athird-stage separation and recovery structure of a polystyrene solutioninstalled thereto. The members corresponding to the second-stageseparation and recovery structure 21 of a polystyrene solution will beidentified by the same reference numerals as used in FIGS. 3 to 5 anddetailed descriptions of them will be omitted.

In the separation and recovery apparatus of a polystyrene solutionaccording to the third embodiment, the die 27 at the end portion on theexhaust side of the second-stage cylinder member 22 of the separationand recovery structure 21 of a polystyrene solution is replaced by acylinder member 32 of the third-stage separation and recovery structure31 of a polystyrene solution which is connected to the end portion atright angles. The molten polystyrene discharged from the second-stagecylinder member 22 is made to enter into the cylinder member 32 from theupstream of the third-stage cylinder member 32 in the lengthwisedirection thereof.

The third-stage separation and recovery structure 31 of a polystyrenesolution is, similar to the second-stage separation and recoverystructure 21 of a polystyrene solution, composed mainly of the cylindermember 32, an mixing shaft (screw shaft) 34 being inserted in thecylinder member 32 and having a spiral mixing impeller 33, a pluralityof heaters 25 attached to the outer peripheral surface of the cylindermember 32 along the lengthwise direction thereof, and a driving deviceof the mixing shaft 34.

Since the driving device of the mixing shaft 34 has the same structureas that of the driving device of the mixing shafts 4 and 24 of thefirst-stage and second-stage separation and recovery apparatuses 1 and21 of a polystyrene solution, it is not illustrated in detail.

This separation and recovery apparatus of a polystyrene solution isdesigned to have a higher solvent separating capacity than that of theseparation and recovery apparatus of a polystyrene solution according tothe second embodiment. The mixing impeller 33 has a spiral form and themolten polystyrene flowing into the third-stage cylinder member 32 fromthe second-stage cylinder member 22 can be whittled away by the spiralmixing impeller 33 continuously.

The third-stage cylinder member 32 has an inner diameter large enoughnot to fill it with the molten polystyrene flowing into the third-stagecylinder member 32 from the second-stage cylinder member 22. This canincrease the surface area of the molten polystyrene and can promote thevaporization of the solvent.

The gas generated by the vaporization of the solvent is discharged byits own pressure through the vent tube 36 disposed on the upstream ofthe connection between the third-stage cylinder member 32 and thesecond-stage cylinder member 22. Alternatively, it is suctioned by avacuum pump.

The molten polystyrene after vaporization of the solvent can advance inthe third-stage cylinder member 32 and extruded outside from a pluralityof through-holes of the die 37 disposed at the tip of the cylinder 32 asstrands.

In these embodiments, as described above, the separation and recoveryapparatus of a polystyrene solution can be used in the recycling methodof expanded polystyrene according to the present invention, comprisingsteps of reducing a volume of the expanded polystyrene; dissolving thevolume-reduced expanded polystyrene in a solvent; and separating andremoving the solvent from the solution of the expanded polystyrene andextruding the expanded polystyrene. These apparatuses can be used notonly for the recycling method of the present invention, but also for theseparation of the polystyrene solution into polystyrene and solvent andextrusion of polystyrene.

EXAMPLES

The present invention will hereinafter be described in detail byExamples and Comparative Examples. It should however be noted that thepresent invention is not limited to or by the following examples.

Example 1 Volume Reduction by Mechanical Compression+Dissolution Methodby Solvent

An expanded polystyrene molded or formed product was crushed into a sizeof an egg, followed by volume reduction of the expanded polystyrenemolded or formed product by compression using a screw type extruderequipped with a conical cylinder having, at the tip portion thereof, aninternal diameter of 2.5 cm. The resin extruded from the tip of theextruder had an apparent density of 0.2 g/cm³ after the volume reductionby compression, with one outside portion being molten by the frictionalheat in the extruder but a large portion being unmelted. The resultingvolume-reduced resin was poured in methylene chloride (temperature: 15°C.) filled in a container equipped with a mixer to dissolve the resinunder atmospheric pressure. The resin was dissolved while emittingbubbles in the container and finally, a solution having a resinconcentration of 40 weight % was obtained. The bubbles emitted duringthe dissolution were released to the air from the nozzle of thecontainer. The amount of methylene chloride which was released to theair together with the gas was determined by weighing the solution beforeand after the dissolution. The solution thus obtained was fed to theseparation and recovery apparatus of a polystyrene solution according tothe first embodiment which had therein an exhaust port for dischargingthe vapor of methylene chloride. By heating with a heater placed at theouter cylinder member of the extruder, the methylene chloride wasremoved by evaporation. The highly-viscous molten resin was extrudedfrom the die attached to the separation and recovery apparatus of apolystyrene solution according to the first embodiment, whereby recycledpellets were produced. The temperature of the extruder was, in thevicinity of the tip, 180° C. at the maximum.

Example 2 Volume Reduction with Partial Melting+Dissolution Method bySolvent

By using a similar extruder to that used in Example 1, expandedpolystyrene was extruded and partially molten under external heating ata temperature of 180° C. As a result, a resin ingot containing manybubbles therein and had been, for the most part, volume-reduced bycompression in the unmelted form was obtained from the tip of theextruder. The resin ingot had an apparent density of 0.4 g/cm³.

The resin ingot was then dissolved in methylene chloride. The resultingmethylene chloride solution had a resin concentration of about 40 weight%. The solution was treated in a similar manner to Example 1, wherebyrecycled pellets were produced.

Comparative Example 1 Heat Melting Method

Expanded polystyrene was thrown to a screw extruder. Recycled pelletswere produced by melting the expanded polystyrene by heating with aheater disposed at an outer cylinder member, extruding the molten resininto strands having a diameter of 2 mm from the dies of a porous plateat the tip of the extruder, and cutting the strands by a pelletizer.Sufficient heating time in the extruder was necessary in order todissolve the polystyrene as complete as possible and leave no bubbles inthe resin. The recycled pellets had a density of about 1.0 g/cm³.

Comparative Example 2 Dissolution Method by Solvent

An expanded polystyrene molding was crushed into a proper size anddissolved in methylene chloride as in Example 1. At the initial stage ofthe dissolution, the expanded polystyrene dissolved actively in thesolvent while emitting a large amount of bubbles of an expanding gas,compared with Example 1. With a rise in the resin concentration,however, the dissolution rate lowered gradually. At the end, a solutionhaving a resin concentration of about 40 weight % was obtained. Theresulting solution was treated in a similar manner to Example 1, wherebyrecycled pellets were produced.

The results of the lost amount of methylene chloride and weight averagemolecular weight of the recycled pellets as measured by GPC weredescribed below. The lost amount of methylene chloride indicated belowwas the amount per kg of the resin dissolved therein. Weight averageLost amount molecular of methylene Example weight chloride Ex. 1 Volumereduction by 287 thousand 5 g mechanical compression + dissolutionmethod by solvent Ex. 2 Volume reduction with 250 thousand 3 g partialmelting + dissolution method by solvent Comp. Ex. 1 Heat melting method185 thousand — Comp. Ex. 2 Dissolution method by 293 thousand 50 g solvent Control Expanded polystyrene as 305 thousand — raw material

From the above-described results, it has been found that the recycledpellets obtained in Comparative Example 1 had a weight average molecularweight lower than that corresponding to the strength usually requiredfor recycled pellets (200 to 250 thousand). This suggests that therecycled pellets obtained in Comparative Example 1 have a problem in thequality suited for recycling use of expanded polystyrene; resins havinga high weight average molecular weight and therefore showing a strengthsufficient for practical use were produced in Example 1 by using volumereduction by mechanical compression and dissolution by solvent incombination or in Example 2 by using volume reduction by compressionwith partial melting and dissolution by solvent in combination. Theabove-described results have revealed that recycled expanded polystyrenehaving a high weight average molecular weight and therefore being suitedfor practical use can be obtained by the method using volume reductionand dissolution method by solvent in combination.

The recycled pellets obtained in Comparative Example 2 had no problem intheir quality. However, the amount of the vapor of the solvent releasedout of the apparatus together with an expanding gas in the volumereducing step is considerably great compared with that in Example 1.This leads to such disadvantages as an increase in the consumptionamount of the solvent or requirement of a large apparatus for recoveringthe solvent from the released gas.

Example 3

In a similar manner to Example 1 except that 0.5 weight % of propyleneoxide and 0.01 weight % of 2-pentene were added as additives tomethylene chloride serving as a solvent to dissolve expanded polystyrenetherein, recycled pellets were produced. The vapor of the solventremoved by evaporation in the separation and recovery apparatus of apolystyrene solution according to the first embodiment was subjected tocold condensation. The concentration of hydrogen chloride existing inthe condensate was measured. As a control, the concentration of hydrogenchlorine when the solvent was composed solely of methylene chloride wasmeasured similarly. As a result, when the solution contained onlymethylene chloride, the concentration of hydrogen chloride was 0.012weight %, while when propylene oxide and 2-pentene were added to thesolution, the concentration of hydrogen chloride was 0.0003 weight %,suggesting that the addition of the additives prevented thedeteriorating decomposition of methylene chloride.

INDUSTRIAL APPLICABILITY

As described above in detail, the present invention enables efficientrecycling of expanded polystyrene waste while alleviating the problemssuch as difficulty in the removal of foreign matters, reducing of themolecular weight owing to heating in the extrusion step, and moreoverdischarge of the vapor of the solvent out of the apparatus together witha gas emitted during the solvent-dissolving and volume-reduction stepand loss of the solvent due to the discharge.

The present invention also makes it possible to prevent, by addingadditives to methylene chloride, which is a suitable solvent fordissolving expanded polystyrene therein, deterioration and decompositionof the solvent due to heat and thereby promoting recycling use thereof.The recycling use of the solvent enables suppression of the whole costof this recycling method and actualizes economical recycling.

In addition, the volume reducing step and dissolving step can be carriedout at different sites so that a station for the volume reducing stepwhich does not use a solvent and therefore needs simple management canbe disposed near the place where the expanded polystyrene waste occurs.Expanded polystyrene which has been made suitable for transport by thevolume reduction can be gathered in a plant where the dissolving step isperformed without transporting the solvent and the polystyrene. In thedissolving step which needs management of solvents, economy of scale canbe gained easily by gathering a large amount of expanded polystyrene andtreating them en masse. Accordingly, the efficiency of recyclingtreatment of expanded polystyrene waste shows a drastic improvement.

By the separation and recovery apparatus of a polystyrene solutionaccording to the present invention, the solvent can be vaporizedefficiently from the polystyrene solution and polystyrene can beobtained stably by extrusion while minimizing the remaining amount ofthe solvent in the polystyrene resin.

1. A method for recycling expanded polystyrene comprising steps of:reducing a volume of the expanded polystyrene; dissolving thevolume-reduced expanded polystyrene in a solvent; and extruding thedissolved expanded polystyrene.
 2. The method according to claim 1,further comprising a step of producing recycled expanded polystyrenefrom the extruded product.
 3. The method according to claim 1, whereinthe volume reducing step comprises volume-reduction by mechanicalcompression and/or volume-reduction by partial melting at a temperaturenot greater than 200° C.
 4. The method according to claim 1, wherein thesolvent used in the dissolving step has a boiling point not greater than150° C.
 5. The method according to claim 1, wherein the solvent used inthe dissolving step is methylene chloride.
 6. The method according toclaim 5, wherein the solvent further comprises unsaturated hydrocarbonhaving 5 to 7 carbon atoms and/or epoxide.
 7. The method according toclaim 1, further comprising a first transportation step for transportingthe volume-reduced expanded polystyrene after the volume reducing stepto carry out the dissolving step at another place.
 8. The methodaccording to claim 7, further comprising a second transportation stepfor transporting the extruded product after the extruding step toproduce recycled expanded polystyrene at another place.
 9. The methodaccording to claim 8, further comprising, after the secondtransportation step, a step of impregnating the extrusion product withan expanding agent and a third transportation step for transporting theproduct.
 10. A separation and recovery apparatus of a polystyrenesolution wherein the polystyrene solution obtained by dissolvingexpanded polystyrene in a solvent is made to advance in a cylinder whilebeing heated, so that the solvent is vaporized, the polystyrene isseparated from the solvent and the solvent is recovered, comprising: amixing shaft disposed in the cylinder, comprising mixing impellers beingadjacent each other in a shaft direction and being shifted in acircumferential direction.
 11. A separation and recovery apparatus of apolystyrene solution wherein the polystyrene solution obtained bydissolving expanded polystyrene in a solvent is made to advance in acylinder while being heated, so that the solvent is vaporized, thepolystyrene is separated from the solvent and the solvent is recovered,comprising: a cylinder comprising a first-stage cylinder member and asecond-stage cylinder member; and an mixing shaft disposed in thefirst-stage cylinder member, comprising mixing impellers being adjacenteach other in a shaft direction and being shifted in a circumferentialdirection, wherein the second-stage cylinder member comprising a screwshaft is disposed at right angles with the first-stage cylinder member.12. A separation and recovery apparatus of a polystyrene solutionwherein the polystyrene solution obtained by dissolving expandedpolystyrene in a solvent is made to advance in a cylinder while heating,so that the solvent is vaporized, the polystyrene is separated from thesolvent and the solvent is recovered, comprising: a cylinder comprisinga first-stage cylinder member, a second-stage cylinder member, and athird-stage cylinder member; an mixing shaft disposed in the first-stagecylinder member, comprising mixing impellers being adjacent each otherin a shaft direction and being shifted in a circumferential direction,wherein the second-stage cylinder member comprising a screw shaft isdisposed at right angles with the first-stage cylinder member, and thethird-stage cylinder member comprising a screw shaft is disposed atright angles with the second-stage cylinder member.
 13. The methodaccording to claim 2, wherein the volume reducing step comprisesvolume-reduction by mechanical compression and/or volume-reduction bypartial melting at a temperature not greater than 200° C.
 14. The methodaccording to claim 3, wherein the solvent used in the dissolving stephas a boiling point not greater than 150° C.
 15. The method according toclaim 7, wherein the solvent used in the dissolving step has a boilingpoint not greater than 150° C.
 16. The method according to claim 8,wherein the solvent used in the dissolving step has a boiling point notgreater than 150° C.